This proposal seeks continued support to study the molecular details of tRNA recognition by cognate proteins. Four complimentary lines of attack will be followed: (1) Photocrosslinking combined with radioactive postlabelling will be used to define that exact residues in the contact surface between various tRNA isoacceptors and their cognate ligases. The increased sensitivity of these methods will permit similar studies of tRNA complexes with proteins in scant supply such as initiation factors. (2) NMR spectroscopy will be used to monitor the chemical environment and motional parameters of the CCA terminal residues using the enzymatically inserted site specific isotopic probes, 13C or 1H in otherwise fully deuterated tRNA. (3) Collaborative biochemical experiments will be continued with Drs. Wool and Gross to test hypotheses derived from the recently solved crystal structure of the eukaryotic initiator tRNA concerning mode of interaction of the tRNAs and ribosomal nucleic acids. (4) With a refined model in hand and excellent phases available, X-ray crystallographic studies will continue in order to examine codon-anticodon interactions in yeast tRNA Metf. Attempts to produce better crystals of class III tRNA will continue as will efforts to crystallize tRNA: aminoacyl-tRNA complexes.